The present invention relates to dielectric fluid for cooling and electrically insulating x-ray tubes, and more particularly, to a system and method for catalytic hydrogenation of x-ray tube dielectric fluid that is subject to chemical breakdown due to exposure to x-ray radiation.
A dielectric oil is typical fluid used to cool and electrically insulate an x-ray tube. The dielectric oil is subject to chemical breakdown, however, upon exposure to x-ray radiation. After exposure to x-rays, the dielectric oil comprises unsaturated hydrocarbon molecules, free hydrogen atoms, and H2 gas. The formation of the H2 gas is disadvantageous as it may reduce the electrical insulating characteristics of the dielectric oil and may interfere with the transmission of the x-rays. Thus, it is desirable to reduce and/or eliminate the formation of H2 gas in the x-ray tube dielectric fluid.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises dual electrodes of an electrical circuit in a vacuum chamber within a cylindrical vacuum vessel envelope. The vacuum vessel envelope typically comprises a glass tube or a cylinder made of metal. One of the electrodes is a cathode assembly which is positioned in a spaced relationship to a rotating, disc-shaped target that comprises the anode assembly. Upon energization of the electrical circuit connecting the electrodes, the cathode assembly produces a supply of electrons which are accelerated and focused to a thin beam. The thin beam of very high velocity electrons is directed parallel to the axis of the vacuum vessel envelope to strike a section of the rotating target anode. The kinetic energy produced by the beam of electrons striking the surface of the section of the target anode, which comprises a material such as a refractory metal, is converted to electromagnetic waves of very high frequency. These high frequency electromagnetic waves are x-rays. The surface of the target anode is typically angled, which helps to direct the x-rays out the side of the vacuum vessel envelope. After exiting the vacuum vessel envelope, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or for inspecting the contents of luggage at airports.
The x-ray generating device is ordinarily surrounded by a casing filled with a circulating fluid, which helps to minimize the operating temperature of the x-ray tube by absorbing heat. Dielectric fluid for x-ray generating devices typically operates at temperatures in the range of about 20-70xc2x0 C. This very high operating temperature is the result of the thermal energy transferred from the tube to the fluid due to the high electric current required to generate and accelerate the electrons, the kinetic energy produced by the electrons hitting the target, and the x-rays themselves. Dielectric oil is typically the fluid utilized to carry the heat away from the x-ray tube, as dielectric oil can absorb and carry away a large amount of thermal energy.
The circulating fluid used to cool the x-ray tube additionally has dielectric properties that electrically insulate the tube. A typical x-ray tube utilizes a tremendous amount of energy to generate x-rays. A typical x-ray tube may require from about 120,000 to 140,000 volts and from about 40-400 milliamps, which produces up to about 40 kilowatts of power. Whereas this very high electrical charge exists within the x-ray tube, the casing is at ground potential. Without an electrical insulator between the tube and the casing, the electrical charge within the tube would tend to arc to the casing, similar to lightning arcing from the clouds to the earth. So, if there is a bad dielectric insulator around the tube, the voltage can break through the tube and ground to the casing. The break through of the voltage can result not only in the charring of the circulating dielectric, but also in the cracking of the vacuum envelope of the tube. Thus, the dielectric properties of the circulating fluid must be maintained to insure the reliability of the x-ray tube.
The dielectric properties of the circulating fluid, however, are negatively affected by the x-rays generated by the tube. The x-ray radiation breaks chemical bonds within the dielectric fluid. Typically, the x-ray radiation breaks carbon-carbon (Cxe2x80x94C) and carbon-hydrogen (Cxe2x80x94H) bonds, resulting in the release of hydrogen atoms. The free hydrogen atoms combine into diatomic hydrogen or H2, which is a gas that forms bubbles within the circulating dielectric fluid. As the amount of H2 in the dielectric fluid increases, the size of the bubbles can increase and displace the dielectric fluid. The high voltage within the x-ray tube can then arc across the bubble and short out on the casing. Thus, the formation of gas bubbles caused by the break down of the dielectric fluid by the x-ray radiation inhibits the electrical insulating properties of the dielectric fluid, possibly leading to high voltage arcing and the failure of the x-ray tube.
Many sources of gas within the dielectric fluid can be removed by vacuum treating the fluid prior to its use. In this case, however, the gas is produced during the x-ray generating process. As such, vacuum treating the dielectric fluid prior to its use will not eliminate this problem. Thus, there is a need for a method to eliminate the gas produced within the dielectric fluid during the x-ray process.
According to the present invention, a method for hydrogenating a dielectric fluid comprising a hydrocarbon that upon exposure to x-rays releases hydrogen atoms, comprises exposing the dielectric fluid to an effective amount of a catalyst system that promotes the recombination of the hydrogen atoms with the hydrocarbon. The dielectric fluid is employed as a cooling element for an x-ray generating device, and preferably comprises hydrogenated napthacene. The catalyst system operates in temperatures in the range of about 10-300xc2x0 C. and pressures in the range of about 0.1-30 atmospheres. The catalyst system may comprise either a solid, non-soluble catalyst or a soluble catalyst.
A suitable solid catalyst may comprise a Group VIII element or a compound of a Group VIII element. The effective amount of solid catalyst is at least 1 cm2 surface area per liter of the dielectric fluid up to about 100 cm2, and preferably 10 cm2 surface area per liter of the dielectric fluid. The solid catalyst may comprise an element selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or more preferably solid catalyst comprises at least one of palladium and platinum.
A suitable soluble catalyst is in solution with the dielectric fluid, wherein the effective amount of soluble catalyst is at least 0.01 gram per liter of the dielectric fluid up to about 1 gram per liter of dielectric fluid. The soluble catalyst may comprise tris(triphenylphosphine) rhodium (I) chloride, precious metals in solution such as HRu(C2H4)(C6H4PPh2)(PPh3)2), Wilkinson""s catalyst which comprises a rhodium, chromium, phosphorus triphenyl chloride compound, and other similar compounds.
In another embodiment, a system for hydrogenating dielectric fluid subject to the formation of hydrogen gas and unsaturated hydrocarbons due to x-ray exposure from an x-ray generating device, comprises an effective amount of a catalyst system positioned within the x-ray generating device to interact with the dielectric fluid for promoting the reaction of the hydrogen gas with the unsaturated hydrocarbons within the dielectric fluid to reduce the amount of hydrogen gas in the dielectric fluid. The hydrogenating system preferably comprises an x-ray system. The catalyst system operates in temperatures in the range of about 10-300xc2x0 C. and pressures in the range of about 0.1-30 atmospheres. The catalyst system may comprise a solid catalyst or a soluble catalyst. A suitable solid catalyst comprises a Group VIII element or a compound of a Group VIII element. The solid catalyst may comprise an element selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or more preferably solid catalyst comprises at least one of palladium and platinum. The effective amount of solid catalyst is at least 1 cm2 surface area per liter of the dielectric fluid up to about 100 cm2, and preferably 10 cm2 surface area per liter of the dielectric fluid. A suitable soluble catalyst is in solution with the dielectric fluid, wherein the effective amount of catalyst is at least 0.01 gram per liter of the dielectric fluid up to about 1 gram per liter of dielectric fluid. The soluble catalyst may comprise tris(triphenylphosphine) rhodium (I) chloride, precious metals in solution such as HRu(C2H4)(C6H4PPh2)(PPh3)2), Wilkinson""s catalyst which comprises a rhodium, chromium, phosphorus triphenyl chloride compound, and other similar compounds.
In yet another embodiment, an x-ray system, comprises an x-ray generating device for producing x-rays, a dielectric fluid circulated about the device to cool and electrically insulate the device, wherein the fluid comprises a hydrocarbon that upon exposure to the x-rays releases hydrogen atoms, and an effective amount of a catalyst system, in communication with the dielectric fluid, that promotes the recombination of the hydrogen atoms with the hydrocarbon. The catalyst system operates in temperatures in the range of about 10-300xc2x0 C. and pressures in the range of about 0.1-30 atmospheres. The catalyst system may comprise a solid catalyst or a soluble catalyst. A suitable solid catalyst comprises a Group VIII element or a compound of a Group VIII element. The solid catalyst may comprise an element selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or more preferably solid catalyst comprises at least one of palladium and platinum. The effective amount of solid catalyst is at least 1 cm2 surface area per liter of the dielectric fluid up to about 100 cm2, and preferably 10 cm2 surface area per liter of the dielectric fluid. A suitable soluble catalyst is in solution with the dielectric fluid, wherein the effective amount of catalyst is at least 0.01 gram per liter of the dielectric fluid up to about 1 gram per liter of dielectric fluid. The soluble catalyst may comprise tris(triphenylphosphine) rhodium (I) chloride, precious metals in solution such as HRu(C2H4)(C6H4PPh2)(PPh3)2), Wilkinson""s catalyst which comprises a rhodium, chromium, phosphorus triphenyl chloride compound, and other similar compounds.
Finally, the present invention discloses a dielectric fluid comprising a hydrocarbon component, a hydrogenating catalyst system, and wherein the dielectric fluid is suitable for use as a cooling element for an x-ray generating device. The dielectric fluid further comprises about 99.7% hydrocarbon, about 0.1% catalyst system, and the remainder comprising conditioning additives. The hydrocarbon comprises about 99.7% hydrogenated light naphthenic petroleum distillates. The hydrogenating catalyst system comprises tris(triphenylphosphine) rhodium (I) chloride, precious metals in solution such as HRu(C2H4)(C6H4PPh2)(PPh3)2), Wilkinson""s catalyst which comprises a rhodium, chromium, phosphorus triphenyl chloride compound, and other similar compounds.